The present invention is directed to a compact air conditioning system which uses air as the refrigerant medium and is capable of providing both heating and cooling. More specifically, the invention is directed to an exhaust driven air cycle air conditioner for use in a motor vehicle.
Air cycle air conditioners have been proposed for some years as an alternative to vapour-compression air conditions in residential, or similar, applications; for example, U.S. Pat. No. 4,295,518 and European Patent No. EP 0045144-A2. This is because they have several advantages over vapour-compression systems. Firstly, because the evaporators of vapour-compression devices operate at or near 5.degree. C., their heating capacity is seriously impaired at low ambient temperatures, since they rely on heat transfer from ambient to the evaporator. Secondly, in cold weather ice forms on the evaporator heat transfer surfaces thereby increasing pressure losses and also reducing evaporator effectiveness. Thirdly, vapour-compression devices use chlorofluorocarbons (CFCs) as the refrigerant which presents manufacturing and maintenance problems with regard to refrigerant leakage, problems which are substantially reduced when air is used as the refrigerant. In addition, CFCs are known to have a harmful effect on the Earth's ozone layer and it is thought that they may also be contributing to global warming caused by the co-called greenhouse effect.
Despite these advantages, air cycle air conditioning system have yet to make a significant impact on the residential market. This is largely due to their poor coefficient of performance (C.O.P.) relative to CFC based equipment, but also because air cycle designs have previously been based on conventional electric motors as prime movers and are therefore relatively low speed, and hence large, devices. Prior art air cycle air conditioners do often incorporation regenerative heat exchangers for claimed improvements in C.O.P. However, the improvement is minimal when the increased losses due to the presence of the heat exchanger(s) are taken into account, and may also be seen as expendable when offset against the attendant increases in complexity, size and cost of the conditioning system.
A recognized way of significantly improving the C.O.P. of air cycle devices is to operate in reverse flow mode, i.e. the refrigerant air flows through the turbine, rather than the compressor, prior to the heat exchanger. However, this approach is not common in the prior art, probably because the reduced cycle temperatures can create icing problems in the heat exchanger.
In prior art air cycle air conditioners the air delivery temperature is dictated by the ambient and operating conditions, which may be undesirable in some applications from the point of view of safety and/or comfort. Such air conditioners would therefore be improved by incorporating some means of air delivery temperature control.